The present invention relates generally to semiconductor switching devices and more particularly to such a devices based on insulated gate bipolar transistor (IGBT) technology.
The ideal power switching device would be characterized by fast switching speed, low on-resistance, and ease of control. Known power devices include thyristors, which provide very low on-resistance, but require special techniques for control; insulated gate bipolar transistors (IGBTs), which are designed to avoid latch latch-up and are thus easier to turn on and turn off, but have a higher on-resistance than thyristors; and insulated gate field effect transistors (IGFETs), which are faster than IGBTs but have a higher on-resistance. IGFETs are usually referred to as MOSFETs (metal-oxide-semiconductor field effect transistors), even though most modern IGFETs have polysilicon rather than metal gates.
Thyristors are regenerative devices, which means that they latch on as long as there is a current flowing that is above a certain holding current. A regular thyristor (or SCR) cannot be turned off unless the current that passes is less than the holding current. There have been efforts to create a MOS-controlled thyristor, but most reported devices are actually thyristors to which MOSFETs have been added. Such devices typically utilize a MOSFET that is turned on in order to provide an "emitter" short that turns off the thyristor.
The idea of using a MOSFET to turn off a thyristor (or SCR) was introduced by V. A. K. Temple of General Electric. The concept required two gates for the device, one for turn-on (the usual SCR gate) and one for turn-off (the MOSFET gate). However, manufacturing the device required the complex task of merging crude thyristor-like semiconductor processing with the high purity, fine geometry requirements of MOSFET processing. Merging these two processes resulted in a MOS-controlled thyristor process that was complex, low yielding, and with more masking and diffusion steps than are required for a thyristor or MOSFET separately.